Assertion: Cubical void is formed in simple cubic unit cell.

Reason: In simple cubic unit cell, atoms are present only at corners.

Assertion and Reason both are correct statements and reason is the correct explanation for assertion.

Cubical voids 

It is formed by close packing of eight corner atoms in a simple cubic unit cell.

Coordination number = 8

It is surrounded by eight spheres located at the corners of a cube.

Cubical void is located at body center of a simple cube.